Multidisplinary study of near seafloor hydrothermal circulations at Lucky Strike, Mid-Atlantic Ridge.
Abstract
Mid-ocean ridge black smoker fluids are modified in the highly permeable domain that lies within a few hundred meters of the seafloor. Mixing with seawater, chemical reactions and geomicrobial activity lead to the formation of localized diffuse effluents. Our purpose is to characterize these diffuse venting areas at the Lucky Strike hydrothermal vent field (Mid-Atlantic Ridge), to understand the structure and dynamic of near surface hydrothermal processes and their forcing role on the fauna.
We use geological observations, seafloor mapping, and time series of fluid chemistry and temperature, current and seafloor pressure at two representative hydrothermal sites, the mature Tour Eiffel site and the younger White Castle site. These sites are semi-elliptical, 30 to 70m in diameter, with black smokers surrounded by two types of diffuse vents: venting cracks and sandy patches. Fluid chemistry shows that effluents in both cases result from mixing of seawater with end-member fluids and reveals a local and temporal variability of processes such as heating of the seawater component, cooling of the end-member fluid, anhydrite precipitation or dissolution. Temperature time-series show that cracks and sandy patches effluents have different dynamics. Cracks show episodic perturbations and short period variations of up to ~20°C; sandy patches have more stable effluent temperatures with long-term (up to 5-month) variations. Probes distant by only a few meters in both cracks and patches show unrelated records. Semi-diurnal frequencies are present in most fluid temperature time-series, particularly at venting cracks, with amplitudes of up to 2°C. The correlation between exit-fluids temperature and seawater pressure at tidal frequencies varies with time with weekly to monthly periods of higher/lower coherency. Tidal cycles therefore also impact fluids circulation in the substratum. These results reveal the complex and time-variable permeability structure of the substratum. They also show differences between venting cracks and diffuse patches as habitats for the hydrothermal fauna: cracks are impacted by large amplitude episodic variations, and the relative stability and moderate temperature of fluids venting in sandy patches probably also affect the near-seafloor chemosynthetic fauna.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMV043...10W
- Keywords:
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- 8416 Mid-oceanic ridge processes;
- VOLCANOLOGY;
- 8419 Volcano monitoring;
- VOLCANOLOGY;
- 8427 Subaqueous volcanism;
- VOLCANOLOGY;
- 8494 Instruments and techniques;
- VOLCANOLOGY